1. Field of the Invention
The present invention relates to a aqueous ink, in particular, a aqueous ink suitable for ink-jet recording. The present invention also relates to an ink-jet recording method, a recording unit, an ink cartridge, an ink set, and an ink-jet recording apparatus.
2. Description of the Related Art
Regarding the ink-jet recording system, recording is performed by generating ink droplets by various ink ejection system, and by attaching the ink droplets to a recording material, for example, paper, so as to form ink dots. Examples of the ink ejection system include an electrostatic attraction system due to application of high voltage, a system in which mechanical vibration or displacement is applied to ink using a piezoelectric element, and a system in which the pressure of bubbling due to heating of ink. According to the ink-jet recording method, noise is reduced, and high-speed printing and multicolor printing can be performed.
Inks used for this method are required to perform so that, for example, images produced with the inks have high definition and have no feathering and no bleeding, and have excellent fastness, water resistance, rub resistance, etc.
In particular, a black ink should preferably be able to print characters which are excellent in quality, such as sharpness thereof, optical density and reproducibility of black color, since the primary use of the black ink is printing characters.
However, the black ink using a water-soluble dye is insufficient in water resistance and reproducibility of black color. Therefore, carbon black has been used as the coloring material. In order to use the carbon black as a water-soluble ink, however, it is necessary to stably disperse it in an aqueous medium. In Japanese Patent Laid-Open No. 3-210373, a method in which a water-soluble resin is used as a dispersing agent has been disclosed. As described above, in general, dispersion has been performed using the dispersing agent.
However, when inks using the aforementioned dispersing agents are used for ink-jet recording, clogs and non-ejection of the inks may occur depending on the resins constituting the dispersing agents. Furthermore, since aqueous pigment inks using the dispersing agents are viscous, there has been a problem in that sometimes, ejection became unstable during a long term of continuous ejection and high-speed printing.
On the other hand, in order to solve the aforementioned problems, dispersion methods without using dispersion agents have been disclosed in Japanese Patent Laid-Open No. 5-186704, Japanese Patent Laid-Open No. 8-3498, International Patent Publications WO96/18688, WO96/18689, and WO96/18696. These are methods in which water-soluble groups are introduced on the surface of the carbon black so as to stably disperse in aqueous media. Self-dispersing carbon black is cationically or anionically charged depending on the hydrophilic group on the surface thereof, has dispersibility in water due to repulsion of the ions thereof, and has improved hydrophilicity due to the hydrophilic group. As a consequence, the pigment ink stably dispersed in an aqueous medium during a long period of storage can be produced. An aqueous black ink using this carbon black as the coloring material provides excellent character quality and high image density, and the formed image has excellent water resistance, rub resistance, etc.
Among the aforementioned publications, in International Patent Publications WO96/18688, WO96/18689, and WO96/18696, the carbon black having a cationic hydrophilic group is disclosed. In particular, ink for ink-jet using cationic carbon black as a coloring material is disclosed in International Patent Publication WO96/18695. Furthermore, it is disclosed in Japanese Patent Laid-Open No. 10-183046 that bleeding between colors can be prevented by the combination of a black ink using the aforementioned cationic carbon black as a coloring agent and a color ink including an anionic material.
In the aforementioned publications, for example, International Patent Publication WO96/18696, it is described that an ink including self-dispersing carbon black containing an amine as a surface functional group is preferably made to be acidic in order to stabilize the dispersion state of the carbon black in the water.
In the present invention, it was discovered that not only in an ink including self-dispersing carbon black having an amine as a surface functional group, but also in an ink including self-dispersing carbon black having a cationic group as a surface functional group, the dispersibility of the self-dispersing carbon black in the ink could be further stabilized by controlling the pH thereof to be on the acidic side. When the pH of an ink including the cationic self-dispersing carbon black was controlled to be acidic, dispersibility of the carbon black was further stabilized. The reason for this is believed that the cationicity of the carbon black is stably maintained in the ink, and coagulation is not likely to occur due to electrical repulsive forces between the carbon black particles.
According to further research leading to the present invention, it was made clear that there was a technical problem that had to be solved in order to further stably form high quality images using an ink including cationic self-dispersing black. That is, when documents and images having blank portions are printed using the aforementioned ink, there were periods during which ejection of the ink from a nozzle is temporarily suspended in response to the blank. Thereafter, at the time of resumption of ink ejection, there were sometimes problems in that the ink to be ejected was not ejected, and ink ejection was fluctuated.
Accordingly, it is an object of the present invention to provide aqueous ink, which exhibits characteristics based on cationic self-dispersing carbon black, that is, for example, in an ink-jet recording apparatus, an image has high character quality and high image density, and furthermore, bleeding between anionic color ink and the aqueous ink is reduced, and wherein the ink has superior ejection stability even after temporary suspension of printing.
It is also an object of the present invention to provide an ink-jet recording method which can stably produce high quality prints.
It is also an object of the present invention to provide an ink-jet recording apparatus, a recording unit, and an ink cartridge, which can stably form high quality prints.
It is also an object of the present invention to provide an ink set which can be used for forming high quality prints.
The present invention is based on research on various inks in consideration of the aforementioned objects. Consequently, it was discovered that by using an aqueous pigment ink as black ink, wherein the ink includes a cationic self-dispersing pigment, in which at least one cationic group bound to the surface of the pigment directly or with another atomic group therebetween, a water-soluble organic solvent, and water, and the ink contains an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1, high character quality and high image density could be achieved, and superior ejection stability could be achieved even after temporary suspension of printing, while the characteristic was maintained, that is, bleeding between anionic color ink and the aqueous ink was reduced, so that the present invention has been made.
According to an aspect of the present invention, there is provided an aqueous ink comprising: a cationic self-dispersing pigment, in which at least one cationic group is bound to the surface of the pigment directly or with another atomic group therebetween; a water-soluble organic solvent; and water, wherein the aqueous ink comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
According to another aspect of the present invention, there is provided an aqueous ink comprising cationic self-dispersing carbon black, in which at least one cationic group is bound to the surface of the carbon black directly or with another atomic group therebetween, a water-soluble organic solvent, and water, wherein the aqueous ink comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
According to another aspect of the present invention, there is provided an ink-jet recording method comprising a step of performing recording on a recording material by ejecting an ink from an orifice in response to a recording signal is provided, wherein the ink is an aqueous ink comprising cationic self-dispersing carbon black, in which at least one cationic group is bound to the surface of the carbon black directly or with another atomic group therebetween, a water-soluble organic solvent, and water, and wherein the aqueous ink further comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
According to another aspect of the present invention, there is provided a recording unit comprising an ink container containing an ink and a head for ejecting the ink as ink droplets, wherein the ink is an aqueous ink containing cationic self-dispersing carbon black, in which at least one cationic group is bound to the surface of the carbon black directly or with another atomic group therebetween, a water-soluble organic solvent, and water, and wherein the aqueous ink further comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
According to another aspect of the present invention, there is provided an ink cartridge comprising an ink container containing the aforementioned ink.
According to another aspect of the present invention, there is provided an ink-jet recording apparatus comprising a recording unit comprising an ink container containing an ink and a head for ejecting the ink, wherein the ink is an aqueous ink containing cationic self-dispersing carbon black, in which at least one cationic group is bound to the surface of the carbon black directly or with another atomic group therebetween, a water-soluble organic solvent, and water, and wherein the aqueous ink further comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
According to another aspect of the present invention, there is provided an ink-jet recording apparatus comprising a recording head for ejecting the aforementioned ink, and an ink cartridge comprising an ink container containing the ink.
According to another aspect of the present invention, there is provided an ink set comprising a first aqueous ink and a second aqueous ink, wherein the first aqueous ink comprises cationic self-dispersing carbon black, in which at least one cationic group is bound to the surface of the carbon black directly or with another atomic group therebetween, a water-soluble organic solvent, and water, and wherein the first aqueous ink comprises an acidic compound including a proton in the structure thereof that is dissociated due to dissolution into water, and having a dissociation stage of the proton with a dissociation constant of less than 1.
Herein, in the case that the second aqueous ink containing an anionic coloring material is used, reaction can occur between the first and second inks by contacting the first and second inks in a liquid state on a recording medium, so that fixation of the coloring material can be further improved. When the second aqueous ink is color ink having a color tone different from the first aqueous ink, even if the first aqueous ink and the second aqueous ink are adjacently applied on the recording medium, bleeding at the boundary region therebetween can be effectively prevented.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.